diff --git a/src/qibolab/instruments/qm/config.py b/src/qibolab/instruments/qm/config.py
index f87a6346b9..2f3bfc5a74 100644
--- a/src/qibolab/instruments/qm/config.py
+++ b/src/qibolab/instruments/qm/config.py
@@ -332,7 +332,7 @@ def register_waveform(self, pulse, mode="i"):
                 self.waveforms[serial] = {"type": "constant", "sample": pulse.amplitude}
         else:
             waveform = getattr(pulse, f"envelope_waveform_{mode}")(SAMPLING_RATE)
-            serial = hash(waveform)
+            serial = hash(waveform.tobytes())
             if serial not in self.waveforms:
                 self.waveforms[serial] = {
                     "type": "arbitrary",
diff --git a/tests/test_instruments_qm.py b/tests/test_instruments_qm.py
index b344e08bf4..6caf5c239f 100644
--- a/tests/test_instruments_qm.py
+++ b/tests/test_instruments_qm.py
@@ -285,8 +285,8 @@ def test_qmopx_register_pulse(dummy_qrc, pulse_type, qubit):
             "operation": "control",
             "length": pulse.duration,
             "waveforms": {
-                "I": hash(pulse.envelope_waveform_i()),
-                "Q": hash(pulse.envelope_waveform_q()),
+                "I": hash(pulse.envelope_waveform_i().tobytes()),
+                "Q": hash(pulse.envelope_waveform_q().tobytes()),
             },
         }